Diffraction of Light by Ultra-Sonic Waves

Abstract

IN a recent paper, R. Lucas1 has described an experiment designed to test the relative merits of a theory, proposed by C. V. Raman and N. S. Nagendra Nath, of the scattering of light by high-frequency sound waves and that due to himself and P. Biquard. The two theories differ in that, while Raman and Nath assume only a change in phase of normally incident light as it progresses through the sound field, Lucas and Biquard's theory takes account of changes in both amplitude and phase. Lucas's experiment appears to prove by indirect methods that the theory of Raman and Nath is true only for sound fields of thicknesses of the order of 4 mm. or less. R. Bar2 has also reported on an experiment which indicates that while the Raman-Nath theory predicts a type of intensity distribution which agrees qualitatively with the observed patterns at frequencies of 1·5 and 7·5 megacycles, there are several discrepancies which indicate that the fundamental assumption of these authors is incorrect. The portion of their theory dealing with oblique incidence apparently gives results which are not in accord with the observations, particularly at the higher frequency. Furthermore, Bar has obtained photographs of the actual ultra-sonic grating by the insertion of a photographic plate in an appropriate position in the light beam behind the stationary sound field. With no lens system of any type to focus the light rays, a series of quite sharp fringes appears on the plate, indicating the presence of some changes in light amplitude as well as in phase.